"On tilt" is a poker term used to describe someone who is letting their luck affect the way they play. For example, if someone has lost a bunch of hands in a row he or she might start playing recklessly to try to make up for it. Also, if a player has won many hands in a row, they might start to play differently as well. This playing out-of-line so to speak is called playing "on tilt."

Player: I doubt he has a great hand. He's probably only betting a lot because he's playing on tilt.

To gamble recklessly and aggressively after a bad or improbable beat or series of bad or improbable beats. Usually results in losing all of your money and then some. Good gamblers avoid this at all costs, even if it means going home earlier than expected.